An implantable medical device (IMD) is implanted in a patient to monitor, among other things, electrical activity of a heart and to deliver appropriate electrical therapy, as required. Implantable medical devices include pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (ICD), and the like. The electrical therapy produced by an IMD may include pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to its normal sinus rhythm. These pulses are referred to as stimulus or stimulation pulses.
Some IMDs supply a pacing therapy to hearts to treat ventricular tachycardia and/or ventricular fibrillation. The pacing therapy may include supplying stimulus pulses to the left and right ventricles of the heart at a stimulation rate. Applying the stimulus pulses to the ventricles may restore mechanical synchrony to the heart. For example, the stimulus pulses may return the heart to a normal rate of ventricular contraction.
Pacing therapies of some known IMDs monitor cardiac signals of the heart to determine when to supply stimulus pulses. For example, after detecting a paced or intrinsic cardiac event, such as a ventricular contraction, the IMD continues to monitor the cardiac signals to determine if a subsequent intrinsic cardiac event occurs during a predetermined escape interval after the preceding cardiac event. If no subsequent cardiac event is detected during the predetermined escape interval, then the IMD supplies a stimulus pulse to the heart to induce contraction of the heart.
During ventricular tachycardia or ventricular fibrillation, intrinsic ventricular contractions may not provide the same level of cardiac output as paced ventricular contractions, or ventricular contractions that are induced by the delivery of stimulus pulses to one or more of the ventricles. For example, intrinsic ventricular contractions may pump less blood through the heart when compared to paced ventricular contractions during ventricular tachycardia or ventricular fibrillation.
Some known IMDs increase the stimulation rate at which stimulus pulses are applied to the heart to increase the number of paced contractions and decrease the number of intrinsic contractions. The IMDs monitor intrinsic and paced ventricular contractions during ventricular tachycardia and ventricular fibrillation in order to determine the stimulation rate. The IMDs determine the stimulation rate based on the predetermined escape interval. For example, the IMDs may decrease the escape interval of a ventricle for subsequent cardiac cycles when an intrinsic ventricular contraction is sensed during the escape interval of a current cardiac cycle. The escape interval is decreased to increase the possibility that the escape interval expires and the IMD delivers a stimulus pulse prior to intrinsic ventricular contraction. For example, decreasing the escape interval provides a shorter time in which an intrinsic contraction must occur before the IMD delivers a stimulus pulse to the heart. On the other hand, an IMD may increase the escape interval if the IMD has been pacing the heart for at least a predetermined time period. The IMD increases the escape interval to reduce the stimulation rate and avoid pacing the heart at an accelerated rate for greater than the predetermined time period.
The known IMDs described above do not, however, account for the presence of fusion-based behavior of the heart. Fusion-based behavior of the heart may reduce the cardiac output of the heart. Reducing the cardiac output of the heart during a pacing therapy may prolong the therapy or fail to remedy the ventricular tachycardia or ventricular fibrillation. Fusion-based behavior may include fusion and pseudo-fusion between paced and intrinsic cardiac events. Fusion may occur when a stimulus pulse is applied to a heart chamber at approximately the same time that an intrinsic contraction of the heart chamber begins. Pseudo-fusion may occur when a stimulus pulse is applied to the heart chamber shortly after an intrinsic contraction of the heart chamber begins. By failing to account for the presence of fusion-based behavior while pacing the heart, some known IMDs continue to ineffectively pace the heart.